Coined after iceberg, the British-English noun fatberg denotes a very large mass of solid waste in a sewerage system, consisting especially of cooking fat that has congealed and hardened after being poured down a domestic drain and of personal hygiene products that have been flushed down toilets.

For example, the following is from The Guardian (London and Manchester, England) of 12^th September 2017:

A fatberg weighing the same as 11 double decker buses and stretching the length of two football pitches is blocking a section of London’s ageing sewage network.
The congealed mass of fat, wet wipes and nappies is one of the biggest ever found and would have risked raw sewage flooding on to the streets in Whitechapel, east London, had it not been discovered during a routine inspection earlier this month.
Now workmen armed with shovels and high-powered jets are working seven days a week to break it up. The grim task is expected to take three weeks.

The noun fatberg is first recorded in One man’s junk, by Terry Grimley, published in the Birmingham Post (Birmingham, West Midlands, England) of 22^nd January 2008—here, fatberg denotes a large lump consisting of congealed cooking fat, washed up on a beach:

Terry Grimley meets an artist with an individual take on the throwaway society
Alistair Grant’s exhibition at Mac is rubbish. Or, at least, it’s made up entirely of rubbish.
[…]
The exhibition, The King Shoddy Show, is part of Grant’s larger ongoing project Landskip, which has to do with the threat of rubbish-dumping, both official and unofficial, to Britain’s landscape. Its title is a pun on the fact that the word “landskip” was an 18^th century version of “landscape”.
[…]
“The idea is that in the future Britain will cease to be Britain and will become the Ugly Islands because it’s so overrun with rubbish. Everyone who is wealthy enough will leave and Britain will be sold off as landfill, ruled over by King Shoddy.”
[…]
After an initial display of photographs showing some of Britain’s mounting tide of rubbish in situ (particularly memorable are the large, rock-like lumps of cooking fat Grant calls “fatbergs”, washed up on Newborough Beach on Anglesey), the second space is dedicated to King Shoddy’s art collection, a diverse range of abandoned items accumulated by Grant while working on public art commissions in various parts of the country.

Coined after fatberg, the noun concreteberg denotes a very large mass of concrete in a sewerage system, consisting of cement that has been poured down a drain. The earliest occurrence that I have found is from The Guardian (London and Manchester, England) of 18^th April 2019:

People pouring concrete into sewers has led to a “concreteberg” forming in central London that weighs 105 tonnes, as heavy as a blue whale.
The 100-metre-long mass is blocking three Victorian-era sewers in the heart of the capital. […]
[…]
Sewers are more commonly afflicted by fatbergs, congealed masses formed by a mix of non-biodegradable solid matter, such as wet wipes. They have become a problem in England, the result of an ageing Victorian sewer network and a rise in the usage of disposable or flushable cloths.
The concreteberg is believed to have been caused by a construction company pouring surplus cement down a drain.

The ‘concreteberg’ found under the streets of central London. Photograph: Thames Water: